.TH std::ranges::nth_element 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::ranges::nth_element \- std::ranges::nth_element

.SH Synopsis
   Defined in header <algorithm>
   Call signature
   template< std::random_access_iterator I, std::sentinel_for<I> S,

             class Comp = ranges::less, class Proj = std::identity >
   requires std::sortable<I, Comp, Proj>                              \fB(1)\fP \fI(since C++20)\fP
   constexpr I

       nth_element( I first, I nth, S last, Comp comp = {}, Proj proj
   = {} );
   template< ranges::random_access_range R,

             class Comp = ranges::less, class Proj = std::identity >
   requires std::sortable<iterator_t<R>, Comp, Proj>                  \fB(2)\fP \fI(since C++20)\fP
   constexpr ranges::borrowed_iterator_t<R>

       nth_element( R&& r, iterator_t<R> nth, Comp comp = {}, Proj
   proj = {} );

   Reorders the elements in [first, last) such that:

     * The element pointed at by nth is changed to whatever element would occur in that
       position if [first, last) were sorted with respect to comp and proj.
     * All of the elements before this new nth element are less than or equal to the
       elements after the new nth element. That is, for every iterator i, j in the
       ranges [first, nth), [nth, last) respectively, the expression std::invoke(comp,
       std::invoke(proj, *j), std::invoke(proj, *i)) evaluates to false.
     * If nth == last then the function has no effect.
   1) Elements are compared using the given binary comparison function object comp and
   projection object proj.
   2) Same as \fB(1)\fP, but uses r as the range, as if using ranges::begin(r) as first and
   ranges::end(r) as last.

   The function-like entities described on this page are niebloids, that is:

     * Explicit template argument lists cannot be specified when calling any of them.
     * None of them are visible to argument-dependent lookup.
     * When any of them are found by normal unqualified lookup as the name to the left
       of the function-call operator, argument-dependent lookup is inhibited.

   In practice, they may be implemented as function objects, or with special compiler
   extensions.

.SH Parameters

   first, last - the range of elements to reorder
   r           - the range of elements to reorder
   nth         - the iterator defining the partition point
   comp        - comparator used to compare the projected elements
   proj        - projection to apply to the elements

.SH Return value

   1) An iterator equal to last.
   2) Same as \fB(1)\fP if r is an lvalue or of a borrowed_range type. Otherwise returns
   std::ranges::dangling.

.SH Complexity

   Linear in ranges::distance(first, last) on average.

.SH Notes

   The algorithm used is typically introselect although other selection algorithms with
   suitable average-case complexity are allowed.

.SH Possible implementation

   See also the implementation in msvc stl, libstdc++, and libc++: \fB(1)\fP / \fB(2)\fP.

.SH Example


// Run this code

 #include <algorithm>
 #include <array>
 #include <functional>
 #include <iostream>
 #include <ranges>
 #include <string_view>

 void print(std::string_view rem, std::ranges::input_range auto const& a)
 {
     for (std::cout << rem; const auto e : a)
         std::cout << e << ' ';
     std::cout << '\\n';
 }

 int main()
 {
     std::array v{5, 6, 4, 3, 2, 6, 7, 9, 3};
     print("Before nth_element: ", v);

     std::ranges::nth_element(v, v.begin() + v.size() / 2);
     print("After nth_element:  ", v);
     std::cout << "The median is: " << v[v.size() / 2] << '\\n';

     std::ranges::nth_element(v, v.begin() + 1, std::greater<int>());
     print("After nth_element:  ", v);
     std::cout << "The second largest element is: " << v[1] << '\\n';
     std::cout << "The largest element is: " << v[0] << "\\n\\n";

     using namespace std::literals;
     std::array names
     {
         "Diva"sv, "Cornelius"sv, "Munro"sv, "Rhod"sv,
         "Zorg"sv, "Korben"sv, "Bender"sv, "Leeloo"sv,
     };
     print("Before nth_element: ", names);
     auto fifth_element{std::ranges::next(names.begin(), 4)};
     std::ranges::nth_element(names, fifth_element);
     print("After nth_element:  ", names);
     std::cout << "The 5th element is: " << *fifth_element << '\\n';
 }

.SH Output:

 Before nth_element: 5 6 4 3 2 6 7 9 3
 After nth_element:  2 3 3 4 5 6 6 7 9
 The median is: 5
 After nth_element:  9 7 6 6 5 4 3 3 2
 The second largest element is: 7
 The largest element is: 9

 Before nth_element: Diva Cornelius Munro Rhod Zorg Korben Bender Leeloo
 After nth_element:  Diva Cornelius Bender Korben Leeloo Rhod Munro Zorg
 The 5th element is: Leeloo

.SH See also

   ranges::max_element  returns the largest element in a range
   (C++20)              (niebloid)
   ranges::min_element  returns the smallest element in a range
   (C++20)              (niebloid)
   ranges::partition    divides a range of elements into two groups
   (C++20)              (niebloid)
   ranges::partial_sort sorts the first N elements of a range
   (C++20)              (niebloid)
                        partially sorts the given range making sure that it is
   nth_element          partitioned by the given element
                        \fI(function template)\fP
